Conversations on Natural Philosophy, in which the Elements of that Science are Familiarly Explained Part 46

_Emily._ So that the effect which takes place on the ray entering the gla.s.s, is undone on its quitting it. Or, to express myself more scientifically, when a ray of light pa.s.ses from one medium into another, and through that into the first again, the two refractions being equal, and in opposite directions, no sensible effect is produced.

_Caroline._ I think the effect is very sensible, for, in looking through the gla.s.s of the window, I see objects very much distorted; articles which I know to be straight, appear bent and broken, and sometimes the parts seem to be separated to a distance from each other.

_Mrs. B._ That is because common window gla.s.s is not flat, its whole surface being uneven. Rays from any object, falling upon it under different angles, are, consequently, refracted in various ways, and thus produce the distortion you have observed.

_Emily._ Is it not in consequence of refraction, that the gla.s.ses in common spectacles, magnify objects seen through them?

_Mrs. B._ Yes. Gla.s.ses of this description are called _lenses_; of these, there are several kinds, the names of which it will be necessary for you to learn. Every lens is formed of gla.s.s, ground so as to form a segment of a sphere, on one, or both sides. They are all represented at fig. 1, plate 20. The most common, is the _double convex_ lens, D. This is thick in the middle, and thin at the edges, like common spectacles, or reading gla.s.ses. A B, is a _plano-convex_ lens, being flat on one side, and convex on the other. E is a _double concave_, being, in all respects, the reverse of D. C is a _plano-concave_, flat on one side, and concave on the other. F is called a _meniscus_, or _concavo-convex_, being concave on one, and convex on the other side. A line pa.s.sing through the centre of a lens, is called its _axis_.

_Caroline._ I should like to understand how the rays of light are refracted, by means of a lens.

_Mrs. B._ When parallel rays (fig. 6) fall on a double convex _lens_, that only, which falls in the direction of the axis of the lens, is perpendicular to the surface; the other rays, falling obliquely, are refracted towards the axis, and will meet at a point beyond the lens, called its _focus_.

Of the three rays, A B C, which fall on the lens D E, the rays A and C are refracted in their pa.s.sage through it, to _a_, and _c_; and on quitting the lens, they undergo a second refraction in the same direction, which unites them with the ray B, at the focus F.

_Emily._ And what is the distance of the focus, from the surface of the lens?

_Mrs. B._ The focal distance depends both upon the form of the lens, and on the refracting power of the substance of which it is made: in a gla.s.s lens, both sides of which are equally convex, the focus is situated nearly at the centre of the sphere, of which the surface of the lens forms a portion; it is at the distance, therefore, of the radius of the sphere.

The property of those lenses which have a convex surface, is to collect the rays of light to a focus; and of those which have a concave surface, on the contrary, to disperse them. For the rays A and C, falling on the concave lens X Y, (fig. 7, plate 19.) instead of converging towards the ray B, in the axis of the lens, will each be attracted towards the thick edges of the lens, both on entering and quitting it, and will, therefore, by the first refraction, be made to diverge to _a_, _c_, and by the seconds, to _d_, _e_.

[Ill.u.s.tration: PLATE XX.]

_Caroline._ And lenses which have one side flat, and the other convex, or concave, as A and B, (fig. 1, plate 20.) are, I suppose, less powerful in their refractions?

_Mrs. B._ Yes; the focus of the plano-convex, is at the distance of the diameter of a sphere, of which the convex surface of the lens, forms a portion; as represented in figure 2, plate 20. The three parallel rays, A B C, are brought to a focus by the plano-convex lens, X Y, at F.

_Emily._ You have not explained to us, Mrs. B., how the lens serves to magnify objects.

_Mrs. B._ By turning again to fig. 6, plate 19. you will readily understand this. Let A C, be an object placed before the lens, and suppose it to be seen by an eye at F; the ray from the point A, will be seen in the direction F G, that from C, in the direction F H; the visual angle, therefore, will be greatly increased, and the object must appear larger, in proportion.

I must now explain to you the refraction of a ray of light, by a triangular piece of gla.s.s, called a prism. (Fig. 3.)

_Emily._ The three sides of this gla.s.s are flat; it cannot, therefore, bring the rays to a focus; nor do I suppose that its refraction will be similar to that of a flat pane of gla.s.s, because it has not two sides parallel; I cannot, therefore, conjecture what effect the refraction by a prism, can produce.

_Mrs. B._ The refractions of the ray, both on entering and on quitting the prism, are in the same direction, (Fig. 3.) On entering the prism P, the ray A is refracted from B to C, and on quitting it from C to D. In the first instance it is refracted towards, and in the last, from the perpendicular; each causing it to deviate in the same way, from its original course, A B.

I will show you this by experiment; but for this purpose it will be advisable to close the window-shutters, and admit, through the small aperture, a ray of light, which I shall refract, by means of this prism.

_Caroline._ Oh, what beautiful colours are represented on the opposite wall! There are all the colours of the rainbow, and with a brightness, I never saw equalled. (Fig. 4, plate 20.)

_Emily._ I have seen an effect, in some respects similar to this, produced by the rays of the sun s.h.i.+ning upon gla.s.s l.u.s.tres; but how is it possible that a piece of white gla.s.s can produce such a variety of brilliant colours?

_Mrs. B._ The colours are not formed by the prism, but existed in the ray previously to its refraction.

_Caroline._ Yet, before its refraction, it appeared perfectly white.

_Mrs. B._ The white rays of the sun, are composed of rays, which, when separated, produce all these colours, although when blended together, they appear colourless or white.

Sir Isaac Newton, to whom we are indebted for the most important discoveries respecting light and colours, was the first who divided a white ray of light, and found it to consist of an a.s.semblage of coloured rays, which formed an image upon the wall, such as you now see exhibited, (fig. 4.) in which are displayed the following series of colours: red, orange, yellow, green, blue, indigo, and violet.

_Emily._ But how does a prism separate these coloured rays?

_Mrs. B._ By refraction. It appears that the coloured rays have different degrees of refrangibility; in pa.s.sing through the prism, therefore, they take different directions according to their susceptibility of refraction. The violet rays deviate most from their original course; they appear at one of the ends of the spectrum, A B: contiguous to the violet, are the blue rays, being those which have somewhat less refrangibility; then follow, in succession, the green, yellow, orange, and lastly, the red, which are the least refrangible of the coloured rays.

_Caroline._ I cannot conceive how these colours, mixed together, can become white?

_Mrs. B._ That I cannot pretend to explain: but it is a fact that the union of these colours, in the proportions in which they appear in the spectrum, produce in us the idea of whiteness. If you paint a circular piece of card, in compartments, with these seven colours, as nearly as possible in the proportion, and of the shade exhibited in the spectrum, and whirl it rapidly on a pin, it will appear white; as the velocity of the motion, will have the effect of blending the colours, in the impression which they make upon the eye.

But a more decisive proof of the composition of a white ray is afforded, by reuniting these coloured rays, and forming with them, a ray of white light.

_Caroline._ If you can take a ray of white light to pieces, and put it together again, I shall be quite satisfied.

_Mrs. B._ This can be done by letting the coloured rays, which have been separated by a prism, fall upon a lens, which will converge them to a focus; and if, when thus reunited, we find that they appear white as they did before refraction, I hope you will be convinced that the white rays, are a compound of the several coloured rays. The prism P, you see, (fig. 5.) separates a ray of white light, into seven coloured rays, and the lens L L brings them to a focus at F, where they again appear white.

_Caroline._ You succeed to perfection: this is indeed a most interesting and conclusive experiment.

_Emily._ Yet, Mrs. B., I cannot help thinking, that there may, perhaps, be but three distinct colours in the spectrum, red, yellow, and blue; and that the four others may consist of two of these colours blended together; for, in painting, we find, that by mixing red and yellow, we produce orange; with different proportions of red and blue, we make violet or any shade of purple; and yellow, and blue, form green. Now, it is very natural to suppose, that the refraction of a prism, may not be so perfect as to separate the coloured rays of light completely, and that those which are contiguous, in order of refrangibility, may encroach on each other, and by mixing, produce the intermediate colours, orange, green, violet, and indigo.

_Mrs. B._ Your observation is, I believe, neither quite wrong, nor quite right. Dr. Wollaston, who has performed many experiments on the refraction of light, in a more accurate manner than had been previously done, by receiving a very narrow line of light on a prism, found that it formed a spectrum, consisting of rays of four colours only; but they were not exactly those you have named as primitive colours, for they consisted of red, green, blue, and violet. A very narrow line of yellow was visible, at the limit of the red and green, which Dr. Wollaston attributed to the overlapping of the edges of the red and green light.

_Caroline._ But red and green mixed together, do not produce yellow?

_Mrs. B._ Not in painting; but it may be so in the primitive rays of the spectrum. Dr. Wollaston observed, that, by increasing the breadth of the aperture, by which the line of light was admitted, the s.p.a.ce occupied by each coloured ray in the spectrum, was augmented, in proportion as each portion encroached on the neighbouring colour, and mixed with it; so that the intervention of orange and yellow, between the red and green, is owing, he supposes, to the mixture of these two colours; and the blue is blended on the one side with the green, and on the other with the violet, forming the spectrum, as it was originally observed by Sir Isaac Newton, and which I have just shown you.

The rainbow, which exhibits a series of colours, so a.n.a.logous to those of the spectrum, is formed by the refraction of the sun's rays, in their pa.s.sage through a shower of rain; every drop of which acts as a prism, in separating the coloured rays as they pa.s.s through it; the combined effect of innumerable drops, produces the bow, which you know can be seen, only when there are both rain, and suns.h.i.+ne.

_Emily._ Pray, Mrs. B., cannot the sun's rays be collected to a focus by a lens, in the same manner as they are by a concave mirror?

_Mrs. B._ The same effect in concentrating the rays, is produced by the refraction with a lens, as by the reflection from a concave mirror: in the first, the rays pa.s.s through the gla.s.s and converge to a focus, behind it, in the latter, they are reflected from the mirror, and brought to a focus, before it. A lens, when used for the purpose of collecting the sun's rays, is called a burning gla.s.s. I have before explained to you, the manner in which a convex lens, refracts the rays, and brings them to a focus; (fig. 6, plate 19.) as these rays contain both light and heat, the latter, as well as the former, is refracted; and intense heat, as well as light, will be found in the focal point.

The sun now s.h.i.+nes very bright; if we let the rays fall on this lens, you will perceive the focus.

_Emily._ Oh yes: the point of union of the rays, is very luminous. I will hold a piece of paper in the focus, and see if it will take fire.

The spot of light is extremely brilliant, but the paper does not burn?

_Mrs. B._ Try a piece of brown paper;--that, you see, takes fire almost immediately.

_Caroline._ This is surprising; for the light appeared to s.h.i.+ne more intensely, on the white, than on the brown paper.

_Mrs. B._ The lens collects an equal number of rays to a focus, whether you hold the white or the brown paper, there; but the white paper appears more luminous in the focus, because most of the rays, instead of entering into the paper, are reflected by it; and this is the reason that the paper does not readily take fire: whilst, on the contrary, the brown paper, which absorbs more light and heat than it reflects, soon becomes heated and takes fire.

_Caroline._ This is extremely curious; but why should brown paper, absorb more rays, than white paper?

_Mrs. B._ I am far from being able to give a satisfactory answer to that question. We can form but mere conjecture on this point; it is supposed that the tendency to absorb, or reflect rays, depends on the arrangement of the minute particles of the body, and that this diversity of arrangement renders some bodies susceptible of reflecting one coloured ray, and absorbing the others; whilst other bodies, have a tendency to reflect all the colours, and others again, to absorb them all.

_Emily._ And how do you know which colours bodies have a tendency to reflect, or which to absorb?

_Mrs. B._ Because a body always appears to be of the colour which it reflects; for, as we see only by reflected rays, it can appear of the colour of those rays, only.